The present invention pertains to the connectors, i.e. the pins and boxes, of tubular drilling members such as drill pipe or drill collars. It is especially applicable to the type of drill pipe generally described in U.S. Pat. No. 4,548,431 to Hall et al, and represents an improvement thereover.
Such a pipe has a pin connector at one end and a box connector at the other end, each connector being adapted to mate with a connector similar to that at the opposite end of the pipe--but on another pipe, to form a tool joint. The type of pin connector in question comprises a pin tong section of relatively large outer diameter; a pin base section outwardly adjacent the pin tong section and of smaller outer diameter, whereby a primary, generally axially facing, annular shoulder is defined between the pin tong and pin base sections; a tapered threaded section outwardly adjacent the pin base section; and a pin nose section outwardly adjacent the threaded section and of smaller diameter (by a value c.sub.1) than the small end of the threaded section, the pin nose section having an annular pin end face. The box connector comprises an internal, generally axially facing, annular secondary shoulder sized to align with the pin end face of a similar pipe in a tool joint, a box base section of relatively small diameter outwardly adjacent the secondary shoulder and sized to receive the pin nose section; a tapered threaded section outwardly adjacent the box base section and adapted to mate with the threaded section of the pin connector; a box counterbore section outwardly adjacent the threaded section of the box connector, of larger inner diameter than the trough of the largest thread of the box connector, and sized to receive the pin base section, the box counterbore section having an annular box end face sized to align with the primary shoulder on the pin connector.
The pipe described in the Hall et al patent is designed to form tool joints which can withstand higher torque loading than conventional tool joints. The analysis reflected in this prior patent was largely limited to the stresses and spring-like qualities of the pin base, the box counterbore section and the pin nose. Although testing and finite element analysis were conducted as a means to confirm the accuracy of the stress analysis, and did not reveal any potential problems, at least one thread failure of this type of pipe during service indicated that the torsional strength of the connection can be limited by the length of the threaded sections. However, the geometry of the joint is such that the diameter of the pin nose section, which also effects the torsional strength, is decreased as the thread length is increased, thus creating a sort of dilemma.
It was discovered by later tests that, when joints were designed with the thread length adequate for the smallest anticipated inside diameter, the cross-sectional area of the pin nose at the largest inner diameter available was too small. Larger inside diameters produced a pin nose with inadequate strength compared to the pin base, the box counterbore section and the threads. In other words, the joint was not "balanced."